C:\GTK\lib and C:\GTK\include to the path
should be C:\GTK\bin.
Also make sure you have no spaces in other entries in your path.
Correcting your PATH will probably cure you, but if you still have
troubles you should read the FAQ entry "I cannot import gtk / gdk /
gobject" at http://matplotlib.sourceforge.net/faq.html and if you
still are having trouble after trying what is suggested there and
reading the links, you may want to consider using the wx backend.
Sorry you're having so much trouble; don't give up hope!